求救，小兰数学

davidliuscu

what is the area of a rectangular surface that has length x feet and width y feet?
1, y is the reciprocal of x
2,the perimeter of the rectanggular surface is 5 feet
答案居然是A，不解
求NN帮助

davidliuscu

第二道是(1/2)^5=1/32是对的啊，弹硬币每次都是1/2,这和口袋里摸小球是不同的。

davidliuscu

第一道条件2解出来x=2或1/2,肯定不充分啊，选A是对的。

davidliuscu

3Q

socat123

第一题A显然是对的啊，面积等于xy，x,y互为倒数，面积不就是1了吗

davidliuscu

还有一道：
If a certain coin is flipped, the probability that the coin will land heads is 1/2. If the coin is flipped 5 times, what is the probability that it will land heads up on the first 3 flips and not on the last 2 flips?

答案怎么能是1/32,
不是应该是C53（1/2)的5次方吗？